Exploring 2D materials at surfaces through synchrotron-based core-level photoelectron spectroscopy

The interest in understanding and controlling the properties of two-dimensional materials (2DMs) has fostered in the last years a significant and multidisciplinary research effort involving condensed matter physics and materials science. Although 2DMs have been investigated with a wide set of different experimental and theoretical methodologies, experiments carried out with surface-science based techniques were essential to elucidate many aspects of the properties of this family of materials. In particular, synchrotron-based X-ray photoelectron spectroscopy (XPS) has been playing a central role in casting light on the properties of 2DMs, providing an in-depth and precise characterization of these materials and helping to elucidate many elusive and intricate aspects related to them. XPS was crucial, for example, in understanding the mechanism of growth of several 2DMs at surfaces and in identifying the parameters governing it. Moreover, the chemical sensitivity of this technique is crucial in obtaining knowledge about functionalized 2DMs and in testing their behavior in several model chemical reactions. The achievements accomplished so far in this field have reached a maturity point for which a recap of the milestones is desirable. In this review, we will showcase relevant examples of studies on 2DMs for which synchrotron-based XPS, in combination with other techniques and state-of-the-art theoretical modeling of the electronic structure and of the growth mechanisms, was essential to unravel many aspects connected to the synthesis and properties of 2DMs at surfaces. The results highlighted herein and the methodologies followed to achieve them will serve as a guidance to researchers in testing and comparing their research outcomes and in stimulating further investigations to expand the knowledge of the broad and versatile 2DMs family.